1. Field of the Invention
This invention relates generally to trans- former circuits and more particularly to flux cancelling transformer circuits used in telephone interface equipment.
2. Description of the Related Art
Transformers are used to couple telephones and telephone switching equipment together and provide excellent line isolation, balance, and current feeding. A typical telephone line transformer circuit is shown in FIG. 1.
The circuit in FIG. 1 provides loop current that powers external devices such as telephones and couples AC signals between primary and secondary sides of the transformer. Direct current (DC) signaling is used by a central office (not shown) to control interface circuitry connected to the transformer T1.
Transformer T1 includes primary windings P1 and P2 which are coupled together by a capacitor C1 to provide a series AC path between P1 and P2. Secondary windings S1 and S2 are connected in series and coupled to a secondary circuit such as a telephone device. Windings P1, P2, S1 and S2 typically have equal numbers of turns. Windings P1 and P2 are referred to collectively as the primary winding, while windings S1 and S2 are referred to collectively as the secondary.
A first battery feed resistor R1 is coupled between P1 and a 48 volt DC voltage supply (V-), while a second battery feed resistor R2 is coupled between P2 and ground. Resistors R1 and R2 limit current flowing in the transformer on short loops, and provide common mode isolation for the circuits. Transformer T1 includes a core of magnetic material.
When DC current flows in the primary side of the transformer, excess magnetic flux is generated in the core of T1. The transformer saturates if the excess magnetic flux is not cancelled. Flux saturation causes the transformer to distort or block AC signals passing between the primary and the secondary of the transformer. Transformer T1 must use a large core to prevent flux saturation caused by the direct current flowing through the primary winding. However, large transformers are expensive and add bulk and weight to the telephone system.
Numerous attempts have been made to reduce the size, weight and cost of transformers used in telephone interconnect circuits. The attempts have generally fallen into one of two techniques. The first technique involves generating an external compensation current and using that compensation current in the windings of the transformer to cancel the DC flux effects. An example of this technique is disclosed by Earp in U.S. Pat. No. 4,096,363, issued Jun. 20, 1978.
One problem with external compensation circuits is that the cancellation current is generated externally. Circuit tolerances in the external compensation circuitry generate a cancellation current that may not precisely match the DC loop current. Thus, a large and more expensive transformer is still needed to tolerate the unbalanced DC current.
Another problem with external DC compensation circuits is that in order to minimize the level of current needed to compensate the DC flux, the compensation winding used for flux cancellation must be wound with more turns. Thus, the compensation transformer is bigger and more expensive. The compensation transformer also operates less efficiently due to stray capacitance associated with the additional windings. Semiconductor circuitry used in external compensation circuits adds cost to the system and reduces overall system reliability. Externally generated cancellation current also requires an additional power supply which increases power consumption and heat dissipation in the transformer circuit.
A second technique for cancelling DC flux uses a separate compensation winding. An example of this technique is disclosed by Martin in U.S. Pat. No. 4,607,142, issued Aug. 19, 1986. One problem with this technique is that the extra winding used for flux cancellation adds cost and size to the transformer. To accommodate the extra winding, the transformer must have a bobbin which includes extra pins. These pins make the bobbin more expensive and bulkier.
Separate compensation windings can require a large number of turns in order to keep the compensation current low. This effects the overall performance of the transformer due to factors such as stray capacitance. Yet another problem is that semiconductor circuitry used to drive this extra compensation winding adds cost to the system and reduces overall reliability due to susceptibility to transients.
Accordingly, a need remains for a transformer line circuit that compensates for DC flux effects while overcoming the above-mentioned problems.